This paper reports on an analysis of the relationship of soil moisture to extreme temperatures in Britain, taken from the Hadley Centre general circulation model HadCM3 (1860--2099) and validated using the central England temperature extremes from the overlapping period 1878--2000. By 2100, HadCM3 predicts higher average temperatures and a greater variability, asymmetry, and persistence of warm extremes. A generalized extreme value analysis relates extremes to the periods of low soil moisture in the model. Longer spells of extreme temperature are seen to arise both from the statistical increase in the frequency of extremes and from the extended periods of low soil moisture. As an example, by 2060--2100 the probability of hot spells longer than 7 days with Tmax > 25¿C is predicted to rise from its present value of 2%/summer to 10%/summer for high soil moisture summers and to 80%/summer when the low soil moisture summers are included.